In this paper，though according to the fact that the gas generation intensity of the Xujiahe coal measure source rocks in the middle of Sichuan Basin is relatively low，still a series of large gas fields (reservoirs) have been proved.Aiming at the problems existing in the study of reservoir formation，the geochemical analysis is used on the natural gas and formation water for a detailed study.Based on the geological background，the geological conditions for the formation of water-soluble gas in the Xujiahe Formation in central Sichuan Basin were analyzed in detail.The study shows that the Xujiahe Formation water in the middle Sichuan Basin is much developed，and the multi-layered coal-bed source rocks and multiple sets of sandstones are superimposed on each other.The Xujiahe Formation has good conditions for the forming of water-soluble gas.The formation water in Xujiahe Formation is CaCl2 type，with high salinity.The H and O isotope value are far away from the atmospheric precipitation line，indicating that the formation water has been well preserved.The dissolved natural gas in water can be well preserved together with the formation water.The tectonic movement of the Himalayan period has caused a significant uplift in the central Sichuan area，and the big erosion of the overlying strata.The decompression of the formation water caused the gas releasing from water.The released gas can either add additional natural gas to the existing gas reservoir，or form a free gas pool itself at higher site in the reservoir.After calculation，one cubic meter of formation water can probably release 2.4m3 of methane during Xujiahe Formation uplift.